1. Field of the Invention
The present invention relates to an image reading device for reading information such as letters or figures on a document as electric signals. In particular, the present invention relates to an automatic sheet feeding type image reading device for conveying sheets stacked on a hopper one by one to a reading unit for reading the conveyed sheet.
2. Description of the Related Art
One of the known automatic sheet feeding type image reading devices is shown in FIG. 15 of the attached drawings. This image reading device includes two feed rollers 41 and 9 arranged on a sheet conveying passage on the upstream side and on the downstream side of a reading head 18, and a pick (and sheet separating) roller 1 for picking and conveying sheets 7 stacked on a hopper 42 one by one. There are two types of pick rollers 1, one being arranged separately from the sheet separating roller, and the other having the functions of picking the sheet and separating the sheet. In any case, two feed rollers 41 and 9 are distinct from the pick roller 1.
In the structure having two feed rollers 41 and 9, the sheet 7 picked by the pick roller 1 impinges against the feed roller 41, so the skewing of the sheet 7 is corrected. Two feed rollers 41 and 9 are driven at an identical rotational speed, so that the sheet 7 from its leading edge to its trailing edge is conveyed at a constant speed.
However, with the development of office automation apparatuses, demands for small and inexpensive apparatuses are increasing. To meet such demands, there is proposed an image reading device including only one feed roller 9 on the downstream side of the image reading head 18, the feed roller 41 on the upstream side of the image reading head 18 being omitted, as shown in FIG. 16. Thus, the pick roller 1 must carry out the function of the feed roller 41 on the upstream side of the image reading head 18.
In the image reading device of FIG. 16, the sheets 7 are stacked on the hopper 42, and are picked one by one in the order of the stack from the top to the bottom, due to a frictional entraining force from the pick roller 1. A pick spring 25 and a sheet separating pad 12 are arranged to resiliently contact the surface of the pick roller 1. The pick spring 25 comprises a smooth leaf spring which urges the sheet 7 to the pick roller so that the frictional entraining force from the pick roller 1 is reliably transmitted to the sheet 7. The sheet separating pad 12 contacts the lower surface of the sheet 7 and gives a frictional resistance to the sheet 7 in order to stop a plurality of sheets 7 from passing therethrough.
Information is carried by the sheet 7, and the sheet 7 introduced into the image reading device is read by the reading head 18 when the sheet 7 passes through the reading head 18. After the leading edge of the sheet 7 is pinched by the feed roller 9, the sheet 7 is conveyed by the feed roller 9 and the reading operation is continued. The sheet 7 is then discharged to a stacker 49. A sensor 50 is arranged at a position on the upstream side of the image reading head 18 for detecting the leading and trailing edges of the sheet 7. In this type of the image reading device, the rotational (peripheral) speed of the feed roller 9 is slightly higher than that of the pick roller 1 to provide tension to the sheet 7, and there is a distance or a gap between the successively conveyed sheets 7 while the sheet conveying motor is continuously rotated.
The pick roller 1 is attached to a roller shaft 2, as shown in FIG. 17. FIG. 17 shows an example, in which the pick roller 1 is stopped for a short time between the previously conveyed sheet and the next sheet 7 to be conveyed while the roller shaft 2 is continuously rotated, so that there is a distance between the successively conveyed sheets 7. In this structure, the pick roller 1 is fixed to the roller shaft 2, and the roller shaft 2 has a stepped portion 43 having a D-cut 44 for preventing rotation. A sleeve 45, a clutch spring 46, and a drive gear 6 are arranged on the narrow shaft portion extending from the stepped portion 43, and they are axially fixed by a snap ring 5. The drive gear 6 is rotatable relative to the roller shaft 2, and one end of the clutch spring 46 is fixed to the drive gear 6. The sleeve 45 is fitted on the D-cut 44 and is not rotatable relative to the roller shaft 2. The sleeve 45 has a pawl 47 for engaging with the end of the clutch spring 46. The clutch spring 46 is a coil spring and one end thereof engages with the pawl 47. The arrow "a" indicates the rotational direction of the pick roller 1, and the arrow "b" indicates the travelling direction of the sheet 7.
The drive gear 6 rotates in the direction of the arrow "a" (the rotational direction of the pick roller 1) at a constant speed. According to this rotational direction, the clutch spring 46 is tightened. Therefore, the clutch spring 46 is twisted so that the diameter thereof is reduced, and the clutch spring 46 can transmit the rotation of the drive gear 6 to the roller shaft 2. The pick roller is thus rotated and the sheet 7 is conveyed. When the sheet 7 is pinched by the feed roller 9 (FIG. 16), the pick roller 1 is pulled by the sheet 7 since the rotational speed of the feed roller 9 is faster than that of the pinch roller 1. Thus, the rotational speed of the sleeve 45 becomes slightly higher than that of the drive gear 6, so the clutch spring 46 is gradually loosened. When the trailing edge of the sheet 7 leaves the pick roller 1, the rotation of the pick roller 1 is stopped while the drive gear 6 again tightens the clutch spring 46. Therefore, there is a distance between the successively conveyed sheets 7, corresponding to the stopping of the pick roller 1.
The distance between the successively conveyed sheets 7 changes according to the length of the sheet 7, as can be understood from the above description. The clutch spring 46 is subjected to a stress both in the spring tightening direction and in the spring loosening direction, and an excessive stress acts in the spring loosening direction if the sheet is longer than a standard one.
In the case of the image reading device having only one feed roller 9, as shown in FIG. 16, the sheet 7 is conveyed at a speed corresponding to the rotational (peripheral) speed of the pick roller 1 until the leading edge of the sheet is pinched by the feed roller 9. The sheet sometimes slides relative to the pick roller 1, since the rotational speed of the pick roller 1 is slower than that of the feed roller 9 and load is applied to the sheet 7 from the sheet separating pad 12 and from the subsequent sheet 7. Therefore, the speed of the leading edge of the sheet 7 at the reading head 18 is slower than the speed of the leading edge of the sheet 7 when the latter is pinched by the feed roller 9. Therefore, a drawn out image is produced when the leading portion of the sheet 7 is read by the reading head 18.
In addition, in the case of the image reading device having only one feed roller 9, as shown in FIG. 16, when the sheet skews, there is no means to correct the skewing. In particular, in the structure in which the top sheet 7 is picked by the pick roller 1, the top sheet 7 does not receive the weight of other sheets 7; if the weight of the other sheets is applied to the sheet to be picked, such a weight may resist the skewing of the sheet. Therefore, if an unbalance force acts on the sheet along the width thereof, the skewing of the sheet occurs to a greater extent.